Vacuum interrupters (VIs) are typically used to interrupt medium to high voltage AC currents. For example, VIs may be used to interrupt or switch AC currents of up to several tens of kiloamperes, in medium to high voltage circuits which include reactive elements and loads. The interrupters include a generally cylindrical vacuum envelope surrounding a pair of coaxially aligned separable contact assemblies having opposing contact surfaces. The contact surfaces abut one another in a closed circuit position and are separated to open the circuit. Each electrode assembly is connected to a current carrying terminal post extending outside the vacuum envelope and connecting to an AC circuit.
A metal-vapor arc is typically formed between the contact surfaces when the contacts are moved apart to the open circuit position while current is flowing through the VI. The arcing continues until the current is interrupted. Metal vapor which is produced from the contacts during the arcing condenses back onto the contacts and also onto vapor shields placed between the contact assemblies and the vacuum envelope after the current is extinguished.
Many applications require the VI to withstand a power-frequency (AC) voltage (typically&gt;100 kV RMS), or a basic impulse level (BIL) voltage (typically&gt;200 kV peak), which is impressed across its open contacts. The voltage withstand ability is in general the highest level of applied voltage at which the VI will reliably not suffer an internal breakdown of the cold vacuum gap, with a specified separation between the contacts and a specified waveform of the applied voltage (AC or BIL). Many applications also require the VI to withstand a transient recovery voltage in order to achieve interruption of high-current arcs.